1. Field of the Invention
The present invention relates to a channel equalizer in an HDTV and, more particularly, to an auto-coefficient renewal digital channel equalizer which can renew filter coefficients automatically without separate external control.
2. Discussion of the Related Art
Most conventional digital filters are modeled after a linear system as shown in FIG. 1, which is embodied as one integrated circuit of an enormous size. For instances, up to 288 taps are used when data is 8 bits and filter coefficients are 10 bits.
FIG. 1 illustrates an FIR(Finite Impulse Response) filter, which is a typical digital filter, and FIG. 2 illustrates a detail system of the coefficient storage part shown in FIG. 1.
Referring to FIG. 1, an input data din which is received in a series of data memories 1-1.about.1-N of flipflops, is shifted continuously in the direction of the arrow based on transitions in the level of a system clock. Each of the data din, .sub.1, din, .sub.2, - - - , din, .sub.n stored in the data memories 1--1.about.1-N and each of filter coefficients C.sub.0, C.sub.1, - - - , C.sub.n-1 stored in coefficient memories 2-1.about.2-N are multiplied in respective multipliers 3-1.about.3-N, the multiplied results are added together in an adder 4 for n filter taps. An address signal Address designating a tap of the coefficient memories 2-1.about.2-N is applied from outside of the system for loading a filter coefficient Cin of the coefficient memories 2-1.about.2-N. That is, the digital filter multiplies the externally applied coefficients C.sub.0, C.sub.1, - - - , C.sub.n-1 for the taps and the data shifted tap-by-tap which is stored in the data memories 1-1.about.1-N, respectively. Then the digital filter adds all the results of multiplication for the taps.
Referring to FIG. 2, for conducting an adaptive filtering, two memories 2a and 2b are provided in parallel in each of the coefficient memories 2-1.about.2-N, one of which being one is designated as a working bank and the other one being designed as a shadow bank by applying a bank selecting signal bs1 thereto. That is, if the coefficient C.sub.k, 1 stored in the first memory 2a is selectively produced through a multiplexer MUX1 in response to the bank selecting signal bs1, the second memory 2b, then defined as the shadow bank, can be updated with an address and a coefficient Cin externally. In this instant, an address signal is applied to each of the first and second memories 2a and 2b (i.e., a load enable signal through an address decoder 2c). If coefficients of C.sub.1, 2, C.sub.2, 2, - - - , C.sub.n-1, 2 are renewed with new coefficients for each of the n taps, the bank selecting signal is inverted with change of the working bank and shadow bank. That is, at this time, the second memory becomes the working bank to produce a coefficient C.sub.k, 2 stored therein selectively through the multiplexer MUX1 in response to the bank selecting signal bs1, and to enable the first memory 2b, then defined as the shadow bank, to be updated with new coefficients C.sub.1, 1, C.sub.2, 1, - - - , C.sub.n-1, 1, externally. Thus, the coefficient storage parts 2-1.about.2-N, update their filter coefficients without influencing the present filtering operation. Upon completion of the updating, the role of the coefficient banks of each coefficient memory 2-1.about.2-N is switched to continue the filtering operation using the renewed coefficients.
FIG. 3 illustrates a channel equalizer of LMS(Least Mean Square) algorithm having the filter shown in FIG. 1 applied thereto. In principle, the coefficients for the channel equalizer should be renewed and, at the same time, equalized data should be produced in the same intervals with the intervals of data reception. In an actual hardware design of the channel equalizer, the channel equalizer should be provided with a coefficient renewing part 5 for calculation and renewal of a coefficient, a channel equalizing filter 6 for equalizing an actually received data using the coefficient received from the coefficient renewing part 5, and a controlling part 7 for controlling the operations of the channel equalizing filter 6 and the coefficient renewing part 5. However, the coefficient renewing part 5 can not carry out the coefficient calculating and renewing operations as fast as the rate of the data reception because the channel equalizer, which should renew the coefficients for all taps based on the following equation has only one actual circuit operation as a coefficient calculating part 5c for renewal of the coefficients in the coefficient renewing part 5. EQU C.sub.k-1.sup.j+1 =C.sub.k-1.sup.j +.DELTA.x error x din, .sub.k( 1),
where C.sub.k is a coefficient of a kth tap, .DELTA. is a step size, which is a factor for use in renewing the filter coefficient, error is an external error signal, and din, .sub.k is a data stored in kth tap. That is, in order to satisfy equation (1), the number of the calculating parts 5c which should be provided for renewing the coefficients equals the number of taps in the channel equalizer. However, provision of such number of calculating parts 5 causes difficulty in designing the hardware. Therefore, in order to cope with this difficulty, the coefficient renewing part 5 is adapted to store a predetermined amount of the data received by the channel equalizing filter 6, and reproduce the stored data at a slower rate which is proportional to the number of taps. Then, the coefficient renewing part 5 renews the rest of coefficients using an output of the filter 5d and operates the filter 5d with the renewed coefficients. As one coefficient for one tap can be renewed in one operation, such an operation is repeated a number of times equal to of taps in the filter; the data being processed at a slow rate for satisfying the equation (1). The channel equalizer, having thus calculated the coefficients all of the taps, downloads the coefficients into the shadow banks of the channel equalizing filter 6 according to the bank selecting concept shown in FIG. 2, and, upon renewal of the coefficients throughout the taps, inverts the bank selecting signal to exchange the working bank and the shadow bank of the channel equalizing filter 6. Accordingly, the channel equalizing filter 6 is operated with the renewed coefficients.
In the meantime, according to the GA(Grand Alliance) HDTV standard, a VSB(Vestigial Sideband) transmission system is used for the HDTV transmission system, in which data is transmitted with a data format as shown in FIG. 4. Since a conventional channel equalizer is operated according to the concept shown in FIG. 3, the channel equalizer renews a channel equalizing coefficient of one tap per one symbol. Therefore, because of this operation speed limitation of the channel equalizer, of the one frame data on the 313 lines shown in FIG. 4, the data actually used in the coefficient renewal of the channel equalizer can not exceed an amount of data(260416/n) representing a division of the entire data(260416 symbols) by the number of taps of the channel equalizer(n). For example, if the number of taps of the channel equalizer n=256, the data used in the coefficient renewal of the channel equalizer among data of one frame are 1017(260416/256).
Thus, since the conventional channel equalizer operates based on filter coefficients (Cin) provided from outside the channel equalizer, the conventional channel equalizer is not suitable for application to an adaptive filtering. That is, in the adaptive filtering when an external coefficient renewing function is used, the conventional channel equalizer, should load a coefficient into the filter as soon as it is renewed. Because the conventional channel equalizer requires an additional external memory for storing the coefficient, an additional controlling block for down loading the coefficient stored in the memory into the filter, and a block for elimination of the problem coming from the non-real time coefficient renewal, the conventional channel equalizer has problems in that a circuit size of the equalizer becomes too large and a real time adaptive filtering can not be achieved.